Familiar Letters on Chemistry eBook

Out of those substances which are adapted to the formation
of blood, are formed all the organised tissues.
The other class of substances, in the normal state
of health, serve to support the process of respiration.
The former may be called the plastic elements of nutrition;
the latter, elements of respiration.

Among the former we reckon—­

Vegetable fibrine.

Vegetable albumen.

Vegetable caseine.

Animal flesh.

Animal blood.

Among the elements of respiration in our food, are—­

Fat. Pectine.

Starch. Bassorine.

Gum. Wine.

Cane sugar. Beer.

Grape sugar. Spirits.

Sugar of milk.

The most recent and exact researches have established
as a universal fact, to which nothing yet known is
opposed, that the nitrogenised constituents of vegetable
food have a composition identical with that of the
constituents of the blood.

No nitrogenised compound, the composition of which
differs from that of fibrine, albumen, and caseine,
is capable of supporting the vital process in animals.

The animal organism unquestionably possesses the power
of forming, from the constituents of its blood, the
substance of its membranes and cellular tissue, of
the nerves and brain, and of the organic part of cartilages
and bones. But the blood must be supplied to it
perfect in everything but its form—­that
is, in its chemical composition. If this be not
done, a period is rapidly put to the formation of
blood, and consequently to life.

This consideration enables us easily to explain how
it happens that the tissues yielding gelatine or chondrine,
as, for example, the gelatine of skin or of bones,
are not adapted for the support of the vital process;
for their composition is different from that of fibrine
or albumen. It is obvious that this means nothing
more than that those parts of the animal organism
which form the blood do not possess the power of effecting
a transformation in the arrangement of the elements
of gelatine, or of those tissues which contain it.
The gelatinous tissues, the gelatine of the bones,
the membranes, the cells and the skin suffer, in the
animal body, under the influence of oxygen and moisture,
a progressive alteration; a part of these tissues
is separated, and must be restored from the blood;
but this alteration and restoration are obviously confined
within very narrow limits.

While, in the body of a starving or sick individual,
the fat disappears and the muscular tissue takes once
more the form of blood, we find that the tendons and
membranes retain their natural condition, and the
limbs of the dead body their connections, which depend
on the gelatinous tissues.

On the other hand, we see that the gelatine of bones
devoured by a dog entirely disappears, while only
the bone earth is found in his excrements. The
same is true of man, when fed on food rich in gelatine,
as, for example, strong soup. The gelatine is
not to be found either in the urine or in the faeces,
and consequently must have undergone a change, and
must have served some purpose in the animal economy.
It is clear that the gelatine must be expelled from
the body in a form different from that in which it
was introduced as food.